Short-wavelength atomic-displacement modulation preceding the B2 → B19′ martensitic transformation in a TiNi-based alloy

Abstract

Neutron diffraction studies of the fine structure of a Ti49Ni51 single crystal are performed before the onset of the B2 → B19′ martensitic transformation (temperature of the onset of the transformation Ms=200 K). Upon cooling below 460 K, extra reflections are found to form; their positions in the B2-phase reciprocal lattice correspond to the full non-Lifshitz star of the wave vector q=0.225〈112〉. At temperatures below 430 K, 0.45〈110〉 extra reflections appear, corresponding to second-order diffraction effects. In the temperature range 300–460 K, reversible and nonhysteretic changes in the intensities and positions of both types of extra reflections are observed. Analysis of the intensities and spatial distribution of the extra reflections in the B2-phase reciprocal lattice indicates that a displacive superstructure dominated by longitudinally polarized atomic-displacement waves with q=0.225〈112〉 arises in the single crystal in the pretransition temperature range.